In the art of xerography, a xerographic plate containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind a latent electrostatic image in the non-illuminated areas. This latent electrostatic image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in xerography is illustrated by U.S. Pat. No. 3,121,006 to Middleton and Reynolds which describes a number of binder comprising finely-divided particles of a photoconductive compound dispersed in an electrically insulating organic resin binder. In its present commercial form, the binder layer contains particles of zinc oxide uniformly dispersed in a resin binder and is coated on a paper backing.
In particular examples of binder systems described in Middleton et al., the binder comprises a material which is incapable of transporting injected charge carriers generated by the photoconductor particles for any significant distance. As a result, with the particular materials disclosed in the Middleton et al. patent, the photoconductor particles must be in substantially continuous particle-to-particle contact throughout the layer in order to permit the charge dissipation required for cyclic operation. With the uniform dispersion of photoconductor particles described in Middleton et al., therefore, a relatively high volume concentration of photoconductor, up to about 50 percent or more by volume, is usually necessary in order to obtain sufficient photoconductor particle-to-particle contact for rapid discharge. It has been found, however, that high photoconductor loadings in the binder layers of the resin type result in the physical continuity of the resin being destroyed, thereby significantly reducing the mechanical properties of the binder layer. Layers with high photoconductor loadings are often characterized by a brittle binder layer having little or no flexibility. On the other hand, when the photoconductor concentration is reduced appreciably below about 50 percent by volume, the discharge rate is reduced, making high speed cyclic or repeated imaging difficult or impossible.
U.S. Pat. No. 3,121,007 to Middleton et al. teaches another type of photoconductor which includes a two phase photoconductive binder layer comprising photoconductive insulating particles dispersed in a homogeneous photoconductive insulating matrix. The photoconductor is in the form of a particulate photoconductive inorganic crystalline pigment broadly disclosed as being present in an amount from about 5 to 80 percent by weight. Photodischarge is said to be caused by the combination of charge carriers generated in the photoconductive insulating matrix material and charge carriers injected from the photoconductive crystalline pigment into the photoconductive insulating matrix.
U.S. Pat. No. 3,037,861 to Hoegl et al. teaches that polyvinyl carbazole exhibits some long-wave U. V. sensitivity and suggests that its spectral sensitivity be extended into the visible spectrum by the addition of dye sensitizers. Hoegl et al. further suggests that other additives such as zinc oxide or titanium dioxide may also be used in conjunction with polyvinyl carbazole. In Hoegl et al., it is clear that the polyvinyl carbazole is intended to be used as a photoconductor, with or without additive materials which extend its spectral sensitivity.
In addition to the above, certain specialized layered structures particularly designed for reflex imaging have been proposed. For example, U.S. Pat. No. 3,165,405 to Hoesterey utilizes a two layered zinc oxide binder structure for reflex imaging. The Hoesterey patent utilizes two separate contiguous photoconductive layers having different spectral sensitivities in order to carry out a particular reflex imaging sequence. The Hoesterey device utilizes the properties of multiple photoconductive layers in order to obtain the combined advantages of the separate photoresponse of the respective photoconductive layers.
It can be seen from a review of the conventional composite photoconductive layers cited above, that upon exposure to light, photoconductivity in the layered structure is accomplished by charge transport through the bulk of the photoconductive layer, as in the case of vitreous selenium (and other homogeneous layered modifications). In devices employing photoconductive binder structures, which include inactive electrically insulating resins such as those described in the Middleton et al., U.S. Pat. No. 3,121,006, conductivity or charge transport is accomplished through high loadings of the photoconductive pigment allowing particle-to-particle contact of the photoconductive particles. In the case of photoconductive particles dispersed in a photoconductive matrix, such as illustrated by the Middleton et al., U.S. Pat. No. 3,121,007, photoconductivity occurs through the generation of charge carriers in both the photoconductive matrix and the photoconductor pigment particles.
Although the above patents rely upon distinct mechanisms of discharge throughout the photoconductive layer, they generally suffer from common deficiencies, in that the photoconductive surface during operation is exposed to the surrounding environment, and particularly in the case of cycling xerography, susceptible to abrasion, chemical attack, heat, and multiple exposures to light during cycling. These effects are characterized by a gradual deterioration in the electrical characteristics of the photoconductive layer resulting in the printing out of surface defects and scratches, localized areas of persistent conductivity which fail to retain an electrostatic charge, and high dark discharge.
In addition to the problems noted above, these photoconductive layers require that the photoconductor comprise either a hundred percent of the layer, as in the case of the vitreous selenium layer, or that they preferably contain a high proportion of photoconductive material in the binder configuration. The requirements of a photoconductive layer containing all or a major proportion of a photoconductive material further restricts the physical characteristics of the final plate, drum or belt in that the physical characteristics such as flexibility and adhesion of the photoconductor to a supporting substrate are primarily dictated by the physical properties of the photoconductor, and not by the resin or matrix material which is preferably present in a minor amount.
Another form of composite photosensitive layer which has also been considered by the prior art includes a layer of photoconductive material which is covered with a relatively thick plastic layer and coated on a supporting substrate. U.S. Pat. No. 3,041,166 to Bardeen describes such a configuration in which a transparent plastic material overlays a layer of vitreous selenium which is contained on a supporting substrate. The plastic material is described as one having a long range for charge carriers of the desired polarity. In operation, the free surface of the transparent plastic is electrostatically charged to a given polarity. The device is then exposed to activating radiation which generates a hole-electron pair in the photoconductive layer. The electron moves through the plastic layer and neutralizes a positive charge on the free surface of the plastic layer thereby creating an electrostatic image. Bardeen, however, does not teach any specific plastic materials which will function in this manner, and confines his examples to structures which use a photoconductor material for the top layer.
French Pat. No. 1,577,855 to Herrick et al. describes a special purpose composite photosensitive device adapted for reflex exposure by polarized light. One embodiment which employs a layer of dichroic organic photoconductive particles arrayed in oriented fashion on a supporting substrate and a layer of polyvinyl carbazole formed over the oriented layer of dichroic material. When charged and exposed to light polarized perpendicularly to the orientation of the dichroic layer, the oriented dichroic layer and polyvinyl carbazole layer are both substantially transparent to the initial exposure light. When the polarized light hits the white background of the document being copied, the light is depolarized, reflected back through the device and absorbed by the dichroic photoconductive material. In another embodiment, the dichroic photoconductor is dispersed in oriented fashion throughout the layer of polyvinyl carbazole.
In copending U.S. applications Ser. Nos. 94,139, 93,994, 94,071 and 93,975, all filed on Dec. 1, 1971, photosensitive members having two functional materials are described. The first material is a photoconductor which is capable of photogenerating and injecting photo-excited holes and electrons into a contiguous adjacent electronically active material. The electronically active material comprises a transparent organic material which is substantially non-absorbing to electromagnetic radiation in the region of intended xerographic use, but which is electronically active in that it allows the injection of photo-excited holes or electrons from the photoconductor and further allows these photogenerated charges to be transported through the active material to selectively dissipate a surface charge.
The use of electronically active materials in combination with photoconductors in xerographic photoreceptors, as disclosed in the aforementioned copending applications, is of enormous importance in xerography. For example, this type of photoreceptor can contain as little at 0.1% by volume of photoconductor to electronically active organic material thereby rendering them economical. In addition, these photoreceptors provide outstanding physical strength and flexibility which enable their use under rapid recycling conditions required in present xerographic processes.
Therefore it can readily be seen that there is a continuing need to develop xerographic photoreceptors having the requisite electrical and physical properties for conditions of rapid recycling. In addition, the need for photoreceptors containing novel compositions of electronically active organic materials and photoconductors is readily appreciated.